Update app host

[sbastn]

Updates Strillone-generated links to use https://app.dnsimple.com by default, reflecting the separation between the public site and application hosts.

DNSIMPLE_URL remains available as a deployment override. Test coverage and README configuration documentation have been updated accordingly.

🔍 QA

Scenario: manual test

1. Start strillone locally:

DNSIMPLE_URL=https://app.dnsimple.com go run ./cmd/strillone

2. Create or use an incoming webhook in Slack. Copy the URL, it should be something like https://hooks.slack.com/services/T00000000/B00000000/xxxxxxxx
3. Post to the equivalent local Strillone endpoint:

curl -i \
  -X POST 'http://localhost:4000/slack/T00000000/B00000000/xxxxxxxx' \
  -H 'Content-Type: application/json' \
  --data '{
    "data": {
      "domain": {
        "id": 1,
        "name": "example.com",
        "state": "hosted",
        "account_id": 1010
      }
    },
    "actor": {
      "id": "1",
      "entity": "user",
      "pretty": "example@example.com"
    },
    "account": {
      "id": 1010,
      "display": "Example Account",
      "identifier": "example-account"
    },
    "name": "domain.create",
    "api_version": "v2",
    "request_identifier": "local-app-host-test-001"
  }'

4. Verify that the terminal with Stillone logs the POST and it contains the correct app host:

➜  strillone git:(update-app-host) DNSIMPLE_URL=https://app.dnsimple.com go run ./cmd/strillone
2026/08/05 18:25:43 Starting dnsimple-strillone/
2026/08/05 18:25:43 WebServer listening on :4000...
2026/08/05 18:28:23 POST /slack/T00000/B0000000000
2026/08/05 18:28:23 [event:local-app-host-test-001] [<https://app.dnsimple.com/a/1010/account|Example Account>] example@example.com created the domain <https://app.dnsimple.com/a/1010/domains/example.com|example.com>
2026/08/05 18:28:23 [event:local-app-host-test-001] Sending event to slack https://hooks.slack.com/services/T00000/B00000000

📋 Deployment Pre/Post tasks

N/A

Deployment Verification

• Confirm a production webhook notification links to https://app.dnsimple.com.

[juankuquintana]

QAd

[sbastn]

@dallasread @lokst this PR needs to be merged shortly after we provide the cut on dnsimple.com to app.dnsimple.com